Our body beats to a rhythm of a ticking clock that is thought to stem from the suprachiasmatic nucleus, found in the hypothalamus of our primitive brain. In fact, our body may have several clocks, but the main clock, the one I like to think of as the grandfather clock, which resides in the hypothalamus and can have an effect on the peripheral clocks, is driven by events in the environment (Papagerakis et al., 2014). More likely, is that the clock is light-responsive, and thus the grandfather clock ticks to a circadian beat (Papagerakis et al., 2014).
“How?” you ask, “does this clock have anything to do with teeth.” After adjusting my position on the high-backed desk chair, I tell you that the ticking clock can be known as a biological rhythm, or biorhythm for short. These biorhythms are thought to exert control on much of an organism, from physiology to behaviour to development (Bromage et al., 2009). When growing, your teeth and bone place layers down on a daily basis, but your teeth are extra special. Whereas bone replaces itself and remodels, your teeth are the same teeth you felt cutting at your gums when they first began to push through after your milk teeth (the formal name being deciduous teeth) had departed.
In the teeth, the daily layers put down by the ameloblasts (the enamel forming cells) are known as cross-striations (Bromage et al., 2009). Then, every so many days, like clockwork, a darker line appears, known as a Retzius line. As the retzius line happens every so many days, it is working to a longer period rhythm, unlike the daily cross-striations (Bromage et al., 2009). The number of cross-striations between each Retzius line should be (there are always exceptions to a rule, for instance Mahoney et al. (2016a) found that the number of cross-striations can change on either side of a hypoplastic defect) identical in the same tooth and all teeth (see below though) for an individual, but while a species should show a general consensus to a certain number of cross-striations, each individual can vary by this (Bromage et al., 2009).
What makes cross-striations and retzius lines so scientifically magical is what this means. First off, while retzius lines are a constant, appearing for example, every seven days, you can get other retzius lines that appear due to a stressful event. These stress lines, sometimes known as accentuated striae tend to appear darker or broader than other retzius lines (Antoine et al., 2009). One particular stressful life event, is when you are born, and this is marked on your deciduous teeth and early forming permanent teeth by the neonatal line (Antoine et al., 2009). Other stressful events may be a nutritional deficiency or a fever, for example, which causes accentuated striae to appear, and create visible defects on the surface of the teeth (known as enamel hypoplasia), such as pits or furrows (Antoine et al., 2009).
Secondly, if, as you grow, your teeth are placing down visible layers on a daily basis, this means something important. It means that, if a child dies before their teeth have completed growing, one can count the cross-striations to see how old they were when they died. Antoine et al. (2009) did just this. Using child skeletons where age-at-death was already known, due to coffin plates and parish records, Antoine et al. (2009) counted cross-striations of the children’s teeth. The count of the cross-striations were only out by just a few weeks of the true age of the children (Antoine et al., 2009).
Thirdly, in children, a relationship was found between thicker enamel of deciduous molars and more cross-striations between retzius lines, as well as a relationship between increased bone formation in the humerus and more cross-striations between retzius lines (Mahoney et al., 2016b). This makes sense, as it would appear a natural conclusion to make that it would take a longer duration for more of something to form. Unfortunately, this statement doesn’t stay as simple for long.
For one thing, Bromage et al. (2009) stated that the relationship is inverse in humans i.e. it should take less time to form more. Reid & Ferrell (2006) also find an inverse relationship between the number of cross-striations between retzius lines and the total number of striae counted, which to me makes sense because although each individual human differs they shouldn’t differ too drastically, thus it would make sense that more cross-striations between striae would mean less striae (this would not go against Mahoney et al. (2016b) findings, as well as complimenting Mahoney et al. (2016a) findings that more space should be measured between two retzius lines with more cross-striations between them). However, Reid & Ferrell (2006) do also find that less cross-striations between retzius lines is correlated with a longer time to form the outside enamel of mandibular canines.
That said, Reid & Ferrell (2006) do query whether this correlation is concrete. Mahoney et al. (2016a) found that the number of cross-striations between each retzius line remained the same in a healthy tooth regardless of whether it was forming the thicker enamel in the lateral regions of the tooth, or the “…narrower enamel layers that form towards the end of a crown’s growth period…” (Mahoney et al., 2016a: 6). While Reid & Ferrell (2006) also measured lateral enamel, this point may show that there is still much to be understood about this topic and what this could mean. Especially since Mahoney et al. (2016a) found that the number of cross-striations between retzius lines tend to increase from deciduous to permanent teeth in the same individual as enamel thickness increases from deciduous to permanent. In an individual, whose tooth enamel thickness decreased in the permanent, so did the number of cross-striations decrease (Mahoney et al., 2016a).
Mahoney et al. (2016a) states that each tooth may vary in the number of cross-striations and enamel growth and thickness as different teeth go through different developmental pathways. To discover if this is the case, it would be interesting for a study to cover a variety of teeth from one individual, and if possible, use a skeletal group from one population using dental variety.
Overall, my own thoughts is that it is more rational that thicker enamel should take longer to construct, just as a cathedral takes longer to build than a house. While this may appear a simple thought, I tend to find that while understanding the processes behind biology are complicated, the process itself tends to lend itself to simplicity. In that respect, Mahoney et al. (2016a; 2016b) work appears to fit the general consensus of the Havers-Halberg Oscillation hypothesis (which I hope to cover in a separate post) that states that the underlying long period biorhythm that controls body mass is correlated with retzius lines, in that “…body mass increases are accomplished by slowing developmental rates over longer growth periods” (Bromage et al. 2009: 398) than the inverse relationship found for humans by Bromage et al. (2009). Obviously with more research into this area, we will gain a better and deeper understanding of the role of biorhythms, life history and teeth.
Antoine, D., Hillson, S. & Dean, C. 2009. The developmental clock of dental enamel: a test for the periodicity of prism cross-striations in modern humans and an evaluation of the most likely sources of error in histological studies of this kind. Journal of Anatomy, 214, pp. 45-55
Bromage, T.G., Lacruz, R.S., Hogg, R., Goldman, H.M., McFarlin, S.C., Warshaw, J., Dirks, W., Perez-Ochoa, A., Smolya, I., Enlow, D.H. & Boyde, A. 2009. Lamellar Bone is an Incremental Tissue Reconciling Enamel Rhythms, Body Size, and Organismal Life History. Calcif Tissue Int, 84, pp. 388-404
Mahoney, P., Miszkiewicz, J.J., Pitfield, R., Deter, C. & Guatelli-Steinberg, D. 2016a. Enamel biorhythms of humans and great apes: the Havers-Halberg Oscillation hypothesis reconsidered. Journal of Anatomy, pp. 1-10
Mahoney, P., Miszkiewicz, J.J., Pitfield, R., Schlecht, S.H., Deter, C. & Guatelli-Steinberg, D. 2016b. Biorhythms, deciduous enamel thickness, and primary bone growth: a test of the Havers-Halberg Oscillation hypothesis. Journal of Anatomy, 228, pp. 919-928
Papagerakis, S., Zheng, L., Schnell, S., Sartor, M.A., Somers, E., Marder, W., McAlpin, B., Kim, D., McHugh, J. & Papagerakis, P. 2014. The Circadian Clock in Oral Health and Diseases. J Dent Res, 93 (1), pp. 27-35
Reid, D. & Ferrell, R.J. 2006. The relationship between number of striae of Retzius and their periodicity in imbricational enamel formation. Journal of Human Evolution, 50, pp. 195-202